Sub-frames are used in modern cars as anchoring points for suspension elements and heavy transmission elements. In particular, sub-frames are used in the front of the car for carrying the engine and the front suspension elements. The sub-frame is normally a U-formed or a rectangular closed frame made from steel or aluminium. Normally, the frame is made from a number of structural members that are welded together. In addition, a number of brackets are welded to the frame, in order to provide fastening points for the engine, control arms or steering components. In case of an aluminium construction, the number of components that have to be welded poses a challenge, as the individual components must be produced with very tight tolerances in order to use automatic welding robots. At the welding joints, gaps of maximum 0.2 mm are allowed. These high-precision production methods are of limited value in the finished product, i.e. the sub-frame is produced with tight tolerances even in places where this is unnecessary for the function of the frame, but contributes to a high production cost.
A possible production method according to prior art would be to use a square section profile and produce four different components, and weld these components together into the finished frame. Subsequently, a number of mounting details, like the engine mounting brackets, must be welded to the frame. The machining of the individual components of the frame means that a lot of material is wasted, in a typical example a frame of about 8-8.5 kg finished weight will need about 20 kg raw aluminium material. This means that about 11.5-12 kg is wasted. In addition, the welding seams are potentially weak points as the welding process impairs the structure of the aluminium.
Japanese patent application 58041737 describes a frame for a motor cycle that is manufactured from a multi chamber profile, probably in aluminium. The profile is split at the ends, between the chambers, and bent into a frame. However, this part is supplemented with cast members that are welded to it, as well as a multitude of mounting brackets that are welded or riveted to the frame. Secondary channels are used as reinforcing elements/load carrying elements in the overall frame construction.